What kind of coatings do Protostar secondaries have?
All Protostar secondary mirrors have hybrid metallic-dielectric coatings applied. (Pure dielectric coatings are not practical for Newtonian secondaries because they warp the edges of the surface.) A layer of pure aluminum is applied on top of a very thin bonding layer to improve the adhesion of the metal to the glass. At least two dielectric layers are deposited on top of the aluminum. Dielectrics are very hard transparent materials that improve the reflectivity, but also greatly improve the durability and scratch resistance of the coating. We use a layer of TiO2 (titanium dioxide) and SiO2 (quartz). Due to the very high melting temperatures of these materials, they cannot be applied properly with conventional thermal evaporating equipment, so an electron beam is used to create a more uniform ion cloud in the chamber. The dielectrics are also applied using an ion-assisted deposition (IAD) process. Heavy, inert, ions are accelerated through an electric field, and propelled at the develo
